1. Field of the Invention
The present invention relates to a method of mending a black defect and a white defect in a mask having a phase shift mask pattern.
2. Description of the Prior Art
FIG. 1 is a cross sectional view showing the structure of a phase shift mask pattern of an on-the-phase-shifter type in which a light shielding pattern is disposed on a phase shifter. In FIG. 1, a phase shifter 2 is formed on a mask substrate 1, and a light shielding pattern 3 is disposed on the phase shifter 2. The light shielding pattern 3 is made of metal such as Cr and MoSi or metal alloy. The phase shifter 2 is provided under every other aperture 20 of the light shielding pattern 3. Since the phase shifter 2 shifts the phase of a light by 180 degrees, an alternation of non-phase-shifted light and phase-reversed light results below the apertures 20.
FIG. 2 is a cross sectional view showing the structure of a phase shift mask pattern of an below-the-phase-shifter type in which a light shielding pattern is located under a phase shifter. In FIG. 2, a patterned light shielding film 3 is disposed on a mask substrate 1, and a phase shifter 2 is formed so as to cover every other aperture 21 of the light shielding film 3. The below-the-phase-shifter type phase shift mask shares the same feature with the on-the-phase-shifter type phase shift mask-that light beams through every other aperture 21 have reversed phases.
FIG. 3 is a cross sectional view showing black defect (area) and a white defect (area) as they are found in a phase shift mask of the on-the-phase-shifter type. As shown in FIG. 3, if the phase shifter 2 is found where it is not supposed to be found, that phase shifter 2 is a black defect (area) 4. On the contrary, if the phase shifter 2 is partially lacking and hence is not found where it is supposed to be found, the lacking portion of the phase shifter 2 is a white defect (area) 7.
FIG. 4 is a cross sectional view showing a black defect (area) and a white defect (area) as they are found, in a phase shift mask of the below-the-phase-shifter type. As in a phase shifter of the on-the-phase-shifter type, the phase shifter 2 which is found where it is not supposed to be found is a black defect 4, and the phase shifter 2 which is partially lacked where it is supposed not to be found as such is a white defect 7.
FIGS. 5 and 6 are cross sectional views showing how to correct a black defect an on-the-phase-shifter type mask having a conventional phase shift mask pattern. The black defect 4 as that shown in FIG. 5 is etched away by local irradiation of an ion beam 22 which is applied to an area containing the black defect 4 as shown in FIG. 6. The ion beam 22 is irradiated from a focusing ion beam (FIB) apparatus which is not shown in the drawing. A black defect of a phase shift mask of the on-the-phase-shifter type is mended in the same fashion.
This mending method, however, inherently produces on the surface of the mask substrate 1 an etching defect 11 which has a similar profile to that of the black defect 4. A light beam through the etching defect 11 has a disordered phase, which must be avoided if high resolution optical lithography is desired. Thus, accurate correction of a pattern defect of a phase shift mask is not attainable with the conventional mending method.
As to a white defect, there is no effective mending method available.